Conventionally, regarding electrophotographic photoreceptors, research and development on organic photoreceptors with a photosensitive layer comprised of an organic photoconductive compound as a main constituent have been conducted.
The surface layer of the organic photoreceptor is subjected to external application of electrical and mechanical forces such as corona discharging, toner development, image transfer to a sheet of paper, cleaning process, etc. Accordingly, the durability against those is required. Furthermore, when a photoreceptor is practically employed in a copier or printer, the photoreceptor surface is degraded by products generated by discharging at the electrode, such as ozone, nitrogen oxide (NO.sub.x), etc., and ultraviolet radiation during exposure. Thus, environmental resistance is required.
As one of the technical measures used to minimize such degradation of a photoreceptor, the addition of various degradation minimizing agents and stabilizing agents has been proposed. For example, Japanese Patent Publication Open to Public Inspection No. 58-120260 describes a method in which benzotriazole compounds known as a UV absorber are added; Japanese Patent Publication Open to Public Inspection No. 63-52146 describes a method in which a p-phenylenediamine is added; Japanese Patent Publication Open to Public Inspection No. 63-58455 describes a method in which dihydroquinoline is added as ozone degradation minimizing agents; Japanese Patent Publication Open to Public Inspection Nos. 63-71856, 63-71857, 63-18355, and 3-73255 describe methods in which spiroindane, spirochroman, hindered-amine, and hindered-phenol are added as antioxidants; Japanese Patent Publication Open to Public Inspection Nos. 63-132224, 63-153553, and 63-153554 describe methods in which .alpha.-keto acid or aryl ketone compounds are added.
The additives mentioned above reduce the adverse effects due to ultraviolet radiation, ozone, NO.sub.x, etc. However, residual potential is occasionally degraded. In recent years, in accordance with the increase in sensitivity of photoreceptors, photoreceptors are required to be mounted on copiers and printers exhibiting higher speed and more printing resistance. Furthermore, it has become difficult to remove discharge products around the photoreceptor because of the decrease in dimensions of copiers and printers. Because of the facts mentioned above, the durability of conventional photoreceptors has been found to be insufficient.